The invention concerns coloured aluminium pigments, a process for the production thereof and use thereof.
Aluminium pigments are used widely in coatings as special-effect pigments. The term special-effect pigments is used to denote pigments which have a directed reflection at oriented, metallic or highly light-refractive particles of a predominantly flat configuration (German Standard DIN 55944). They are always of plate-like or flake-like configuration and have very large particle diameters compared with dye pigments. Their optical properties are determined by reflection and interference. Depending on transparency, absorption, thickness, singlelayer or multi-layer structure, the special-effect pigments exhibit a metallic shine, a pearl shine, interference or interference reflection. The main area of use are in the cosmetics and automobile sector, and in addition in colouring plastic materials, paints, leather coatings, in the printing industry and in the ceramic industry. (For a comprehensive representation of the technical background, see W. Ostertag, Nachr. Chem. Techn. Lab. 1994, 9, 849.).
The special-effect pigments which are most frequently used are aluminium flakes and coated mica flakes, wherein aluminium pigments exhibit a typical metal shine and coated mica flakes exhibit a typical pearl shine.
In recent years the need for coloured special-effect pigments has increased greatly. Therefore for example oxide-covered copper and brass flakes, substrates which are coated with transition metal oxides, such as muscovite, phlogopite or glass, guanine single-crystals (fish silver), BiOCl-single crystals, flake-form haematite single-crystals, flake-form phthalocyanines or crushed thin multi-layer films with a Fabry-Perot-structure are used as special-effect pigments.
In order to achieve colour effects inter alia aluminium pigments are also mixed with transparent dye pigments. The colouristic options with that method are however limited insofar as it is not possible in that way to achieve interference effects and therefore the pigments do not have a pearl shine. However, because of the transparency, interference pigments with a pearl shine, which are mostly based on coated mica flakes, have a poorer covering capability than aluminium pigments. Attempts have therefore been made to produce pigments with the good covering capability of aluminium flakes and the colouristic options of interference pigments, by colouring aluminium pigments.
U.S. Pat. No. 4,328,042 and EP-A-0 033 457 describe the production of gold-coloured aluminium pigments by the deposition of iron oxide, wherein iron pentacarbonyl is oxidised with oxygen in a fluidised bed of the aluminium flakes, the bed being produced by fluidisation with inert gas. The disadvantage of that procedure is the very high level of technological expenditure.
U.S. Pat. No. 5,037,475 describes the production of aluminium pigments which are coloured by fixing colour pigments on the metal surface. The colour pigments are fixed by way of carboxyl group-bearing polymers. A protective layer can be applied by polymerisation, to improve the adhesion. However the pigments produced in that way have only a low level of colour intensity.
WO 91/04293 (PCT/US90/05236) describes the colouring of aluminium pigments by fixing poly -coated colour pigments from aqueous solvents on the metal surfaces by way of electrostatic forces. In that situation the coating result depends in a complex manner on the type of aluminium pigment, the nature of the polymer coating on the colour pigments, the solvent composition and the pH-value.
EP-A-0 328 906 discloses titanium dioxide-coated metal pigments, inter alia also aluminium pigments, which are produced by hydrolysis of an organic titanate ester compound, for example tetraisopropoxytitanium, in the presence of the metal flakes which are suspended in an organic medium, at pH of from 4 to 8. Various colour shadescan be achieved with that process by varying the thickness of the titanium dioxide layer. Maintaining specific conditions is crucial for producing the coated pigments. The pH-value must be in the range of from 4 to 8 and the dropping speed used in adding the titanate ester must be in the range of from 1.0.times.10.sup.-7 to 1.0.times.10.sup.-4 mole per minute and m.sup.2 metal surface area. Therefore that process cannot be used on a large technical scale. In addition the coated pigments must be calcined after the drying operation in order to achieve colour effects as a suitable layer structure is achieved only by removal of the water from the metal oxide layer. Because of the low melting point of aluminium however the calcination operation can only be carried out with a very great deal of difficulty when involving coated aluminium pigments.
U.S. Pat. No. 4,978,394 describes the production of titanium dioxide-coated aluminium pigments by chemical vapour deposition (CVD), wherein titanium tetrachloride is reacted at a low level of concentration with water vapour in a fluidised bed in the presence of hot aluminium particles. This process also suffers from the disadvantage of expensive technology.
U.S. Pat. No. 4,158,074 discloses the production of coloured aluminium pigments by coating with a film comprising hydrated aluminium oxide and hydrated metal oxide. The film is produced by the treatment of fine aluminium flakes or plates in an alkaline solution of an iron, nickel, cobalt, zinc or copper salt at elevated temperature at a pH-value of from 8 to 12, that is to say by an electrochemical reaction of the metal salts. In that way it is possible to produce gold-coloured pigments, and by the addition of chelating agents, also black-brown and grey-white pigments.
U.S. Pat. No. 5,261,955 describes a sol-gel-process for the production of coloured metal pigments, wherein the metal flakes or plates are dispersed in a sol of an inorganic salt, for example an aqueous alkaline zirconium oxide sol, the flakes or plates coated with the sol are dispersed after filtration in a solution of an inorganic compound, for example cobalt nitrate, in an organic solvent, and a sol-gel layer is formed on the flakes by heating. The large number of individual steps involved in that process means that a high level of apparatus expenditure is also required.
JP-A-61-130375 discloses a gold-coloured aluminium pigment, produced by the treatment of aluminium powder with dichrcmate, sodium fluoride and surface-active agents in acid solution, drying and subsequent treatment with a fatty acid derivative. Colour shades other than gold cannot be achieved using this method.
German laid-open application (DE-OS) No 41 40 295 describes pigments comprising carrier materials in flake or plate form, preferably mica, which are coated with an inorganic matrix containing metal oxides and/or dye particles in the sub-micrometre range. The operation of coating the substrates is effected out of acid aqueous suspensions by the hydrolysis of metal salts, preferably titanium tetrachloride, in the presence of metal oxide and/or dye particles. However it is not possible to colour aluminium flakes using that process because under those conditions the aluminium particles quickly break down.
U.S. Pat. No. 3,067,052 discloses coloured aluminium particles which are produced by the oxidation of aluminium powder with KMnO.sub.4 -solution, possibly with the addition of a reducing agent. The colour shade of those pigments is golden, possibly also with a greenish or reddish shade, depending on the respective reducing agent used.
DE 25 57 796 discloses a coloured aluminium pigment which is coated with a metal oxide layer containing carbon black as the colour pigment. The proportion of colour pigment is 10% by weight at a maximum. According to that specification, larger amounts of colour pigment have a detrimental effect on shine and colour.
DE 36 17 430 discloses coated coloured pigments, the base pigment comprising mica. Coatings containing colour pigment are not named in that document. The colour effects of the described pigments are produced by interference.
DE 42 23 383 describes metal sulphide-coated substrates, wherein the metal sulphide layer does not contain any colour pigments.
DE 42 23 384 (U.S. Pat. No. 5,374,306) discloses metal oxide-coated aluminium substrates without a colour pigment component. According to that publication, the lowest possible content of carbon, that is to say lubricants and organic impurities, is decisive in regard to achieving good shine effects. The substrates must therefore be pre-treated by heating in an oxygen-bearing atmosphere.
JP-1-110 568 (Patent Abstracts of Japan, Section C, Volume 13 (1989) No 331 (C-622)) discloses aluminium substrates coated with thin oxide layers, without a colour pigment component, the colour effects thereof being produced by interference.
For the stated reasons, the processes for the production of coloured pigments on a mica basis cannot be used to produce aluminium pigments, in respect of which however there is a high level of interest because of the higher covering capability and the metal shine. The known processes for colouring aluminium pigments however afford only few colour shades, predominantly in the gold range, and for the major part are very expensive in terms of apparatus. Therefore there was still a need for aluminium pigments which are coloured in different colour shades, and a need for a process which is simple in terms of apparatus, for the production of those coloured aluminium pigments.